Inshore Navigation

1 The chart and the almanac

Given that there is enough water to float your boat, navigation divides into two main sections. The first is working out where you are. The second is deciding how to move from there to your destination.

In the simplest case of all, you can see your goal, there is no current and no obstacles stand in the way. All you have to do is look at where you want to go and steer straight to it. This will hold good in still water and clear visibility over any moderate distance, but if the haven of your choice is too far away to be discerned with any certainty, you’re going to need a chart to find it successfully. An almanac will also be essential in order to refine raw information delivered by the chart.

Charts

The chart is the core unit of navigation. It’s really only a very detailed map produced to the highest standards of accuracy which shows the coast, the foreshore, the water and the nature of the seabed. It also indicates rocks, shoals and a variety of objects that will help you work your way from one location to the next. Charts come in two essential variants: paper and electronic. Both have a number of sub-groups. All use similar sets of symbols, scales and general conventions.

Part of a British Admiralty Chart produced by the UKHO (United Kingdom Hydrographic Office).

Types of chart

Step into a good chandlery anywhere in the world and you’ll find three sorts of chart on sale.

■ National charting authorities publish paper charts for their own coastline, and often many others too. One of the most prolific is the British Admiralty. If you’re operating in home waters, these are the yardstick by which others are measured. They come in full-sized, commercial grade charts also called ‘Standard Charts’ from an official chart agency, but ‘Leisure Charts’ are also issued. These are full-sized charts sold folded down to around A4 size. Unlike Standard Charts, they may not be completely up to date when sold. Smaller charts are issued in ‘Leisure Folios’ of popular sea areas. These sit more comfortably on a small vessel’s chart table and some carry additional data on the reverse side. Occasionally British Admiralty charts carry blow-up plans of specific harbours.

■ Commercial companies also publish paper charts. For example, Imray cover the whole of the British Isles and much of Europe and the West Indies. Commercially produced charts rely on survey data that is owned by the official authority but they may deliver it in a form that carries certain advantages. While an Admiralty chart is the ‘Bible’, a commercial unit can offer extras such as numerous plans which may save you having to buy additional Admiralty charts to achieve the same coverage. Further useful data may also be found on the back of such charts.

An Imray chart. Note that the colour scheme differs from the Admiralty and that soundings are in vertical typeface. Most things, including chart symbols, look the same, however. The box around Fowey Harbour appears elsewhere on the same chart as a helpful ‘blow-up’ plan.

■ Electronic charts for use in GPS chart plotters are really computer-screen representations of the paper equivalent, but two essentially different types are on offer.

Raster scan charts are literally high-definition scans of a given paper chart. With such a chart, you may be able to magnify it on the screen, but by doing so you will not reveal any additional information. If you need more detail, you must call up a chart with a different scale. Raster charts are most often found on PC chart programs. Their similarity to familiar paper charts makes them popular with many navigators.

Vector charts are typically seen in a stand-alone chart plotter unit, although many PC plotter software plotter programs use them as well. They are built up electronically so they look quite different from the traditional paper equivalent. However, complete information for a given area can be accessed via a single chart chip or CD. This sounds like good value, and so it is, but there is no free lunch at sea, so while everything may be in there somewhere, it is often far from obvious at first sight. To use vector charts effectively, you need to familiarise yourself with their little ways. You can read more about this in Chapter 11.

An electronic vector chart showing part of South Cornwall.

Chart symbols

Many of the symbols on charts are self-explanatory and some of them will be covered in this manual, but anyone venturing out to sea should buy and study the modestly priced booklet called ‘Admiralty Chart number 5011’ (AP5011). This colourful publication lists the whole lot. None but the most conscientious student could ever learn them all – nor do most of us need to – but the best way to become familiar with the common ones is to get out on the water and to use a chart. Those that really matter will soon become second nature.

Soundings

The unattached, italicised figures you see all over the chart are known as ‘soundings’. They give the depth of water at that point, measured in metres and tenths of metres, and they give it when the tide is as low as it will ever be. If the figure appears on a green part of the chart and has a line underneath it (thus 13), it means that at the lowest of low tides that area will be 1.3 metres above sea level. It is called a drying height. A ‘sounding’ in normal typeface whose figures are set vertically generally refers to the height of an adjacent rock or object. Heights are given relative to mean high water springs (see Chapters 6 and 12). Normal typeface soundings that are set at oblique angles on a depth contour line indicate the depth of the contour.

Vertical Chart Datum

Whilst the cartographers have sensibly and safely decided to chart their depths at a very low tide, a need exists for something less vague. In fact, all soundings are measured below (or, in the case of drying heights, above) a level called Chart Datum or, to be fully correct in the GPS era, Vertical Chart Datum. This is not to be confused with the ‘datum to which the chart is drawn’ – its horizontal datum (see Chapter 7). If this sounds perplexing, don’t worry. The two subjects have little or nothing to do with one another and you are in no danger of mixing them up. It’s just an unfortunate coincidence that they carry the same name.

Chart Datum is taken as the level of tide at the very bottom of the lowest tide that can be predicted. Tidal predictions are calculated by reference to the gravitational pull of sun and moon and so the lowest prediction is called the Lowest Astronomical Tide, or LAT. In reality, tidal predictions may be modified by local weather and other factors, but such considerations are best left to more advanced navigators. Choosing LAT as Chart Datum was very wise indeed of the head cartographer. It means that except in certain places far from the UK you’ll generally be unlucky to find less water anywhere than the charted soundings. The tide will only add to these and increase your safety margins as it does so.

Chart scale

All charts are drawn to some scale or other. For any but the shortest trips, a chart with sufficient detail to guide you in and out of harbour will perforce be too small in its area of coverage to include your destination. Typically, you might therefore need one large-scale harbour chart for each end and a smaller-scale passage chart to see you safely from one to the other. Be aware of this and don’t try to navigate on the wrong scale of chart.

When it comes to electronic charts, a vector chart can be zoomed from one scale to another, which solves the issue neatly. A raster scan cannot, so if you’re using these it’s important to ensure you have the scales you need.

This is an Admiralty Leisure Chart which is sold folded to make storage and use easier in small craft. The fold is clear here, and note that it doesn’t affect the clarity of the information.

Chart corrections

Admiralty charts

If you buy Admiralty Standard Charts from an official chart agency, they will be corrected up to the date when you walk out of the shop. Leisure Charts may not be. The Admiralty does all it can, but these charts sit on shelves in folios or folded up, and cannot be attended to in the same way as Standard Charts. This doesn’t mean they aren’t any good. So long as you either make every effort yourself to update them, or navigate appropriately if you do not, you should be fine. The Admiralty regularly publishes new corrections for worldwide charts online. Because there is far more information in these notices than will ever be required by yachtsmen and fishermen in home waters, the Leisure Charts are covered by a dedicated website with free access. This gives all the information needed to update charts currently in production. Correcting charts is satisfying work and to do it once in a while will not only give you up-to-date information and some useful plotting practice, it will also keep you out of the pub for an evening.

A section of a passage chart. Note the lack of detail in the inshore sections, then compare it with the detailed chart of the boxed area labelled ‘Harbour Chart’ shown in the illustration on the previous page.

Commercially published charts

Late corrections are often issued with the chart itself, with ‘stop press’ data available on the publisher’s website. Imray do both.

How up to date are you?

By international convention, all chart corrections are given a number and a year. When you correct your charts, your job is to enter this in the margin at the bottom left-hand corner. If someone else has already done some, you’ll find his or her corrections noted down, and by checking them against the Notices to Mariners on the corrections website you can see which ones are missing. It’s beautiful in its simplicity, so long as you don’t forget to note down those numbers.

Electronic charts

A number of ways exist for dealing with electronic chart corrections. In some cases you buy a new chip with financial incentives from the manufacturer. Some can be sent in for updating. Charts issued on CD may well have CD updates, and so on. Find out about this before investing in a chart system, because disappointment can be your lot if you don’t like the deal later on.

The Notices to Mariners web page. Navigate from here to all you need for up-to-date charts – free!

Reference numbers after the words Notices to Mariners are corrections that have been applied to this